*2.2. Classification of PLES*

There are two types of studies on the classification of PLES. One type classifies different land uses based on the single functions of production space, living space, and ecological space. For example, Hu [21] and others established a classification system for PLES at the scales of urban and rural regions and urban built-up areas by referring to *the Standard of Urban Land Classification and Planning and Construction Land*. According to the dominant function of a single land category, Ma [22] proposed a one-to-one correspondence between the secondary land-use types and PLES, and conducted a classification study on PLES. This classification method simplified the complex relationship between these three spaces, but ignored the complex multiple functions of land. For example, arable land (paddy field and dry land) was uniformly divided into production space, but its ecological function characteristics were not fully considered. The other classification method is mainly based on the multiple functions of land-use. This method extends the function of PLE, lists the land by each separate compound function type, and classifies the land according to the land-use standard on this basis [23]. Starting from the main function of land, and taking other functions into account, Zhang et al. [24] incorporated the concept of ecological land, and constructed a classification system of Production–Living–Ecological land (PLEL) use at a national scale to coordinate the space of PLE land. This method compensates for the shortcoming that land ecological function is not sufficiently considered in land-use classification, and realizes the connection between land function classification, land-use classification, and urban land classification. Thus, this method is widely used in production practice and scientific research related to land function regulations, ecological political achievement assessments, and ecological environmental effects [25–27]. In addition, based on the theoretical connotations of PLEL, Liu et al. [28] analyzed the dialectical relationship between land-use functions and land-use types from the perspective of the complexity of the land PLE multifunctional complex, and constructed a classification standard system for PLEL according to the national classification standard of land-use status. To some extent, this classification method also reflects that PLES is, in essence, a result of the evolution and regional differentiation of the human–Earth relationship system.

Existing research on the identification of mainstream PLES methods can be divided into research using the merge classification method and the quantitative measurement method. The former involves qualitative research, which mainly merges and classifies land-use data based on yearbook data, national land surveys, and remote-sensing image data to identify PLES [29–33]. This method facilitates the connection between land-use function and classification standards, and compensates for the deficiency of ecological functions in land-use classification to some extent. Thus, this method has been widely used in practical fields [34–36]. However, this method produces errors in the recognition results, as it ignores the composite function of space. Moreover, these results will be different under different classification systems. Most of the latter systems focus on quantitative analysis, and mainly use the calculation function group of the spatial function value to establish a measurement system for land-use function, and identify PLES through a quantitative measure of the leading function of land-use [37]. For example, Li et al. [16] integrated the calculation function group of spatial function value based on ecosystem-service value assessment, and quantitatively identified the PLE functions of different value quantities, as well as the distribution of dominant function space. By processing and analyzing POI data, Cao et al. [38] identified, in more detail, the dominant function of land-use at a micro scale, and then, differentiated and delineated the PLES with single and mixed functions in the city. The advantage of the quantitative measurement method is that it can accurately identify the dominant function of PLES. However, it remains difficult to carry out multi-subject integration and multi-scale integrated expression using this method. Moreover, this method is difficult to apply in practice.
